1. Field of the Invention
The present invention relates to a transfer device and an image forming apparatus having the transfer device that transfers an image onto a recording medium at a transfer nip formed between an image bearer and a transfer member contacting the image bearer.
2. Description of the Background Art
In an image forming apparatus which employs an engaging and disengaging mechanism capable of engaging and disengaging an opposed member from an image bearer to create a transfer nip therebetween under a prescribed bias applied from a biasing device, a linear density unevenness called an impact jitter is sometimes induced when a cardboard is used as the recording medium. Because, a load on the image bearer sharply increases and a line speed thereof largely decreases when the cardboard enters the transfer nip.
The Japanese Patent Application Laid Open No. 10-83124 (JP-10-83124-A) typically suppresses the impact jitter with a transfer roller that includes a cylindrical column roller section and shafts integrally and rotatavely protruding from both side ends thereof and rotation cams capable of providing idling rotation around the shafts. The rotation cam includes a convex at a prescribed rotation angular position to collide with one end of a photoconductive member serving as the image bearer in the shaft direction. Such collision forcibly moves the transfer roller apart from the photoconductive member against a bias force applied by a pressing device toward the photoconductive member, so that a shaft distance between the photoconductive member and the transfer roller is adjusted. When the cardboard is used as a recording medium, the shaft distance is broadened and a transfer pressure is decreased or the transfer roller is disengaged therefrom. As a result, the load on the photoconductive member, which is necessarily generated when the cardboard enters, is suppressed.
Although the sharp increase in load can be avoided by broadening the shaft distance as shown in the JP-H10-83124-A, a transfer error is likely induced due to shortage of transfer pressure. To resolve such a problem, the Japanese Patent Application Laid Open No. 06-274051 (JP-H06-274051-A) provides an image forming apparatus in that a transfer roller is disengaged from a photoconductive member by driving a rotation cam and form a small gap between the transfer roller and the photoconductive member prior to entrance of a cardboard as a recording medium into a transfer nip to suppress impact jitter. Subsequently, by stopping an operation of a solenoid after entrance of a tip of the cardboard into the above-described small gap and releasing the forcible movement of the transfer roller, and thereby subjecting the transfer roller to a biasing force of a spring that serves as a pressing device to press the transfer roller against the photoconductive member, the transfer error is suppressed by applying a sufficient transfer pressure to a recording medium during a transfer process.
It is typically known that an adjustment pattern is timely formed at a prescribed position not to be transferred onto a recording medium under a prescribed image formation condition, such as when an image formation performance is checked to stabilize image quality in an image forming apparatus that includes a transfer device that superimposes images formed on multiple image bearers onto an intermediate transfer member and transfers those at once using an opposed member opposed to the intermediate transfer member.
For example, the Japanese Patent Application Laid Open No. 2007-286176 proposes a system that forms an adjustment pattern on a portion corresponding to an interval between sheets and separates and opens a secondary transfer nip in that case.
Further, the Japanese Patent Application Laid Open No. 2009-145778 proposes a system that disengages an opposed member from an intermediate transfer member when an adjustment pattern passes through a secondary transfer nip and opens a shutter of a detection sensor so that separation of the opposed member and opening and closing of the shutter of the detection sensor are synchronized with each other. Yet further, the Japanese Patent Application Laid Open No. 2006-047779 describes a technique in that a voltage having the same polarity as toner is applied to a secondary transfer nip where an intermediate transfer member contacts a secondary transfer member and an adjustment pattern passes therethrough not to transfer the adjustment pattern onto the secondary transfer member. Such a polarity is generally opposite to that applied to execute transferring onto the intermediate transfer member.
However, the above-described conventional techniques cannot obtain a fine image.